The present invention relates to radiological well logging methods and apparatus for investigating the characteristics of subsurface earth formations traversed by a borehole. More particularly, the invention relates to methods and apparatus for measuring porosities and other properties of earth formations in the vicinity of a well borehole by means of neutron well logging techniques.
In the search for liquid hydrocarbons beneath the earth's crust, one of the parameters which must be known about the earth formation is its porosity. The porosity, or fractional volume of fluid filled pore space present around the rock grains comprising the earth formation, is needed both to evaluate the formation's commercial production potential, and also to assist in the interpretation of other logs, such as resisitivity and pulsed neutron logs.
Several techniques have been developed in the prior art to measure earth formation porosity in a borehole environment. One such technique employs a gamma ray source and single, or multiple, detectors to measure the electron density of the earth formations by the amount of gamma ray scattering. Another technique employs an acoustic transmitter and one or more acoustic receivers. The velocity of compressional wave acoustic transmission through the formation from the acoustic transmitter to the receivers is measured and may be directly related to the formation porosity.
A third commercial technique which has been employed in the prior art to measure the porosity of earth formations employs a neutron source and any of several types of neutron or gamma ray detectors. Because the behavior and interactions of neutrons with matter are quite distinct depending upon their energies, neutron populations are generally divided into at least three distinct energy ranges: fast, epithermal, and thermal. Generally speaking, fast neutrons are those with energies around one MeV (within an order of magnitude or so). Epithermal neutrons have energies around one ev. Thermal neutrons are in thermal equilibrium with their environment and have energies around 0.025 ev. Generally, slower moving or less energetic neutrons are more easily captured by the nuclei of materials in the earth formation. Once such neutrons are captured, these nuclei then decay to lower energy states by the emission of energy-characteristic gamma rays.
The neutron sources commonly employed all emit neutrons in the fast energy range, and depending upon the formation constituents into which the neutrons are emitted, these energies will then be attenuated at various rates by interactions with the matter in the formation. Generally speaking, hydrogen is the agent most effective for slowing down neutrons in an earth formation. Hydrogen in the form of water or hydrocarbons is found in the pore spaces of the formation rocks.
A common neutron porosity logging technique is one which employs either a neutron or gamma ray detector which is sensitive to the density of the thermalized neutrons at some point removed from the neutron source. Then, in a formation containing a larger amount of hydrogen than is present in low porosity formations, the neutron distribution is more rapidly slowed down, and is contained in the area of the formation near the source. Therefore, the counting rates in remote thermal neutron sensitive detectors located several inches or more from the source will be suppressed. In lower porosity formations which contain little hydrogen, the source neutrons are able to penetrate farther. Hence, the counting rates in the more remote detector or detectors are increased. This behavior may be directly quantified into a measurement of the porosity by well established procedures.
One of the most common criticisms of neutron well logging is that it is too sensitive to the borehole and the borehole environment. Thus the response of neutron logs, especially thermal neutron logs, is affected by such factors as diameter irregularities of the borehole wall, mudcake irregularities in open-hole logging, irregularities in the cement annulus surrounding the casing in a cased well borehole, the properties of different types of steel casings in cased wells, variations in the properties of different borehole fluids (especially the chlorine content of the fluid, since chlorine has a high absorption cross-section for thermal neutrons), the various different formation lithologies which surround the borehole, the position of the logging instrument in the borehole, and so forth.
A well-known neutron logging method which has been commercially available for many years and which provides good compensation for many of these limitations is the so-called dual-spaced neutron log. (See L. S. Allen, et al., "Dual-Spaced Neutron Logging for Porosity", Geophysics, Vol. 32, February, 1967.) In such a log, the thermal neutron fluxes at two different distances from a source of fast neutrons are measured and the ratio of these flux measurements is taken. This effectively reduces the effects of such factors as eccentricity of location of the logging tool in the borehole, variations in salinity of the liquid in the borehole, variations in salinity of the liquid in the formation, uncertainties in the borehole size, and so forth.
An alternative to measuring porosity with thermal neutrons is to determine porosity from epithermal neutron measurements. Such measurements are advantageous because they are much less sensitive to thermal neutron absorbers. However, due to lower flux rates, they have the statistical disadvantage of furnishing much lower count rates.
A commercially available enhancement to the dual-spaced neutron log, considered of value in certain logging environments, is the so-called dual-porosity dual-spaced neutron log. (See R. R. Davis, et al., "A Dual Porosity CNL Logging System", SPE Paper #10296, October, 1981.) Such a logging method adds a pair of epithermal neutron detectors to the thermal neutron detector pair. Ratios or other data processing techniques are then employed to exploit the insensitivity of epithermal neutrons to the presence of thermal neutron absorbers (elements with large thermal neutron capture cross-sections) such as chlorine. The two separate porosity measurements (thermal and epithermal) which are obtained can then be compared to provide additional information about the formation, such as clay content, gas detection in shaly gas sands, and so forth. Sometimes inferences can also be drawn concerning formation lithology and salinity, especially when the data can be cross-correlated with data from other logs.
An obvious physical disadvantage of tools for performing the dual-porosity dual-spaced neutron log is their excessive physical length. This can pose a problem particularly where several tools are strung together to provide multiple well logging services on the same borehole pass. At the least, this results in greater tool standoff in rugose boreholes. Also, measurements of the epithermal neutron slowing down rate in such tools are conventionally based upon the same data from which the other (e.g., porosity) measurements are drawn. Not only does this require the use of plural detectors, but since the same basic data is used for both (porosity and slowing down length) measurements, these results are necessarily interrelated. If it were possible to derive them simultaneously yet independently in a logging run, while using the same detectors for both, then cross correlations with this or other measurement data could be significantly more valuable.
A need therefore exists for an improved method and apparatus for formation logging in which such logs as dual-porosity dual-spaced neutron logs can be performed more compactly than possible with present tools using four separate neutron detectors. A need also remains for increasing the raw data obtained with this or similar such tools by measuring and determining characteristic neutron lengths, such as the epithermal neutron slowing down length, independently of, or in addition to, such other measurements. Preferably this should be done with the same neutron detectors which are being used for the porosity measurements, and ideally with as little as a single neutron detector.